Rechargeable energy storage devices are generally connected in series to provide voltage to a load. For example, when two rechargeable energy storage devices are connected in series and are charged from a single supply source, a voltage difference between the two rechargeable energy storage devices is mainly caused by: a difference in the capacitance between the two rechargeable energy storage devices; and an insignificant difference in their leakage currents. When a value of the difference in capacitance between the two rechargeable energy storage devices is large, the voltage across the rechargeable energy device with the smaller capacitance value may exceed its rated maximum allowable voltage, which eventually degrades the rechargeable energy storage device's operating time.
Two known methods exist for balancing voltages in rechargeable energy storage devices that are connected in series: passive balancing; and active balancing. In the passive balancing method, a resistor is placed in parallel with each rechargeable energy storage device that is connected in series. The resistance value of each resistor is the same and the resistance value is selected such that the current through the resistors (the bypass current) is much larger than the maximum leakage current of the rechargeable energy storage devices. In the passive balancing method, there is a trade-off between the time to balance the voltages in rechargeable energy storage devices (hereinafter referred to as the balancing time) and the resistor value which controls a bypass current through each rechargeable energy storage device. The balancing time may be reduced at the expense of an increase in the bypass current. When the bypass current is too high, the bypass current may be a burden to an energy source that is limited by the use of rechargeable energy storage devices having a small capacitance. When the bypass current is too low, the rechargeable energy storage devices may be exposed to overvoltage for an extended period of time, which shortens the useful life time of the rechargeable energy storage devices.
There are many known active balancing methods. Some active balancing methods utilize the limited source and sink currents that are inherent to the rechargeable energy storage devices to balance the voltage across each series connected rechargeable energy storage device. A drawback of these active balancing methods is that the limited currents increase the balancing time.
Other active balancing methods use higher current conduction devices in order to shorten the balancing time. However, to avoid oscillation on the output, these active balancing methods utilize a low pass filter, which slows down the response time and increases the balancing time.
Further active balancing methods utilize a microcontroller and software interface to read the voltage across each series connected rechargeable energy storage device before attempting to balance the voltages. A drawback of these active balancing methods is that these methods are generally slow, which results in some of the rechargeable energy storage devices being overcharged before voltage balancing is achieved.
Improvements to methods and systems for voltage balancing of multiple rechargeable energy storage devices that are stacked in series are therefore desirable.